---
title: "Microstructure of steel"
subject: "Civil Engineering Materials"
chapter: 8
key: "semester_3/civil_engineering_materials:microstructure-of-steel"
source: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/microstructure-of-steel
site: BCT Engineering
---

# Microstructure of steel

Civil Engineering Materials · Chapter 8: Metals and Alloys

- Asked in: 2070 Chaitra (old course), 2+6 marks
- Video: [Microstructure Of Steel - understanding the different phases & metastable phases found in steel.](https://www.youtube.com/watch?v=FE40FawJDkg) — James Sword Engineering 
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/microstructure-of-steel
- Whole chapter: https://www.bctengineer.com/md/semester_3/civil_engineering_materials/chapter-8

## Question

Define microstructure examination of steel and describe the different micro structures of steel.

## Short answer

A ground, polished and etched steel sample is viewed at 100–1000× magnification to reveal phases. Key microstructures: ferrite (soft, white), cementite (very hard, white needles), pearlite (lamellar mixture, 0.8% C, dark), martensite (needle‑like, very hard and brittle) and austenite (stable above ~723 °C).

## Answer

**Microscopic (microstructure) examination** is the study of the internal structure of a metal under a microscope. A small sample is cut, ground, polished to a mirror finish, etched with a mild acid (e.g. nital, 2% nitric acid in alcohol) and viewed at 100–1000× magnification. It shows the phases, grain size and shape, which control the strength and hardness of the steel.

#### Microstructures of steel

1. **Ferrite:** Almost pure iron (about 0.02% C) in BCC form. Soft, ductile, weak, magnetic. Appears as light, white grains.
2. **Cementite ($Fe_3C$):** Iron carbide with 6.67% C. Very hard and brittle. Appears as white needles or plates.
3. **Pearlite:** A lamellar (layered) mixture of ferrite and cementite, with 0.8% C. Appears dark in an etched sample, like mother-of-pearl. Fairly strong and tough.
4. **Austenite:** Solid solution of carbon in FCC iron, stable above about 723 °C. Non-magnetic, soft, tough. Exists at room temperature only in special (stainless) steels.
5. **Martensite:** Supersaturated carbon in BCT iron, formed by rapid quenching of austenite. Needle-like, very hard and brittle.
6. **Bainite:** Fine mixture of ferrite and cementite formed by cooling at an intermediate rate. Harder than pearlite and tougher than martensite.
7. **Troostite and sorbite:** Tempered martensite products. Troostite is harder and finer; sorbite is tougher and less hard.

| Structure | Carbon | Property |
|---|---|---|
| Ferrite | ~0.02% | Soft, ductile |
| Cementite | 6.67% | Hard, brittle |
| Pearlite | 0.8% | Strong, tough |
| Martensite | up to 1.5% | Very hard |

Low-carbon steel has ferrite + pearlite; 0.8% C steel is fully pearlite; high-carbon steel has pearlite + cementite.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Preparing a sample for microstructure examination involves:
   A. Cutting, grinding, polishing, etching with mild acid (nital) and viewing under a microscope
   B. Melting and casting it
   C. Stretching until failure
   D. Weighing it
   Answer: A. Etching reveals phases and grain structure. (Key point: Microscopic examination)

2. Pearlite is:
   A. Pure iron in BCC form
   B. A lamellar mixture of ferrite and cementite with 0.8% C
   C. Carbon in BCT iron
   D. Iron carbide only
   Answer: B. Pearlite is layered ferrite and cementite at 0.8% C. (Key point: Pearlite)

3. Which description is correct?
   A. Ferrite: hard, brittle; cementite: soft
   B. Ferrite: soft and ductile; cementite (6.67% C): very hard and brittle; martensite: very hard from rapid quenching
   C. Martensite forms by slow cooling
   D. Austenite is BCC
   Answer: B. Martensite forms when austenite is rapidly quenched. (Key point: Ferrite, cementite, martensite)

4. Which structure does a high-carbon steel (above 0.8% C) have?
   A. Ferrite + pearlite
   B. Fully pearlite
   C. Pearlite + cementite
   D. Ferrite only
   Answer: C. Low-carbon: ferrite + pearlite; 0.8% C: pearlite; high-carbon: pearlite + cementite. (Key point: Structures by carbon)

## Related in other subjects

- [Design philosophies for steel structures](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Adesign-philosophies-steel) — Design of Steel and Timber Structure, chapter 1
- [Steel versus timber structures](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Asteel-vs-timber-advantages) — Design of Steel and Timber Structure, chapter 1
- [Slow cooling of 1.2% C steel](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Ahypereutectoid-steel-12-carbon) — Material Science, chapter 6
- [Heat treatment processes of steel](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Aheat-treatment-of-steels) — Material Science, chapter 6